Process for producing ceramic materials using silicon carbide

ABSTRACT

Process for producing bodies from ceramic materials using silicon carbide, comprising the steps: configuration of fiber-reinforced porous bodies ( 1, 5 ) that consist of carbon on a base ( 2 ) that is inert relative to liquid silicon, the bodies having cavities ( 3 ) that are accessible from the exterior or surface recesses ( 3′ ), and the cavities ( 3 ) being closed at the bottom in the porous bodies or the surface recesses ( 3′ ) together with the base ( 2 ) forming a reservoir that is sealed at the bottom; heating the configuration by introduction of energy to melt the silicon ( 6 ) that is present in the reservoir; and infiltrating the melted silicon in the bodies ( 1, 5 ) and reaction of the silicon with the carbon to form silicon carbide; and use of the thus produced bodies as brake disks and as clutch driving disks.

The invention relates to a process for producing ceramic materials usingsilicon carbide.

Ceramic materials using silicon carbide are produced, for example,through reaction of liquid silicon with carbon. Preference is given touse of porous molded bodies made of carbon (initial bodies) whose shapecorresponds approximately to that of the molded body made of the ceramicmaterial that is to be produced, in the so-called “near net shapeprocess.” In this case, those initial bodies are preferably used thatare reinforced with fibers that are preferably made of carbon. Thelatter essentially are retained in the reaction of silicon with thecarbon of the initial body matrix and so may continue to exert theirreinforcing action. From the prior art, embodiments are known thatensure this retention of the reinforcing fibers through a specialcoating (DE 197 10 105 A1).

The reaction of the liquid silicon with the initial body usually iscarried out so that the silicon, together with the initial body, isheated to the point of melting, the silicon flowing into the initialbody, rising and falling as a result of the capillary action, throughimmersion or through flowing out of applied silicon particles or bodies,which may also be suitably bound.

A fundamental problem here is in a batch operation, the heating andcooling of the workpieces (initial bodies and siliconized molded bodies)up to the melting point of the silicon at about 1420° C. and thesubsequent cooling to room temperature for post-treatment and furtherprocessing. When liquid silicon is introduced into the initial body byway of wicks, the limited wick support surface results in slowpenetration of the liquid silicon and therefore a long reaction period.During the long reaction period, undesirable secondary reactions arealso promoted, of course, in particular the attack of liquid silicon onthe carbon reinforcing fibers. In addition, it is necessary to retain alarger amount of silicon in the melting vessels than is actuallyrequired for infiltration and reaction. The melting vessels (crucibles)must be resistant to liquid silicon over a long period; the protectivecoating required (for example with boron nitride or through lining withgraphite foil, which in turn is coated with boron nitride) istime-consuming and therefore costly. Cleaning of the crucibles after useand before further use of the by-products and wastes formed over anextended dwell time is also time-consuming.

There is therefore a need for a process for producing ceramic materialsusing silicon carbide, a process in which the introduction of therequired silicon is possible in a simpler way and with less expense.

The invention makes available a process in which fiber-reinforced porousbodies that are made of carbon are configured on a base that is inert toliquid silicon, and the bodies have cavities accessible from theexterior or surface recesses, for example in concave form. It isessential for the invention that the cavities in the porous bodies beclosed at the bottom or form such a space—that is sealed at thebottom—with the base, which in addition, together with the surfacerecesses, is referred to as a reservoir, and that may include thisreservoir or a quantity of silicon that is sufficient for the formationof the ceramic material by reaction of the carbon with the silicon. Butit is also possible, according to the invention, to conduct thedescribed process in several stages, after the reaction of the first“filling,” i.e., the first quantity of silicon, the reservoir beingfilled with silicon at least one additional time, and the reaction beingcontinued.

The configuration is heated to the point of melting the silicon that ispresent in the reservoir by the introduction of energy. The meltedsilicon penetrates into the porous bodies—this process is referred to asinfiltration—and reacts with the carbon in the porous bodies to formsilicon carbide. The heating of the configuration is carried outadvantageously through inductive heating or through radiant heating.

When the silicon in the reservoir is melted, the melted siliconpenetrates the entire contact surface in the porous bodies; this resultsin substantially shorter reaction periods.

By preference the base of the configuration preferably consists ofgraphite or a graphite sheet. A layer may be provided on the graphite orthe graphite sheet as a releasing agent; boron nitride is preferred as areleasing agent.

Studies conducted on the basis of this invention have shown that, as aresult of the shorter reaction period, the attack of the liquid siliconon the carbon reinforcing fibers may be considerably reduced or entirelyeliminated, so that a special fiber protection through coating may bereduced or entirely dispensed with. In addition to the larger contactsurface and the resulting faster penetration of the silicon in thecarbon body, the studies also show that the reduction of the attack onthe fibers is also produced by the shorter heating period.

It is therefore essential for the invention that the silicon not beretained, for example, by a surface coating of particles or a compactpiece after the melting and be able to penetrate the carbon body onlyafter a period of delay, as described in the application DE 198 50 468A1; rather, it is preferable that the melted silicon be able topenetrate the carbon body as quickly as possible.

In the production of brake disk rings that consist of ceramic materialsusing silicon carbide, the cylinder ring itself can act as a reservoirin association with the base. Possible contamination by a releasingagent, which is used as a layer on the base, on the surfaces of thebrake disk rings, which act as a friction surface, may be avoided ifbrake disk rings with a molded-on pot of the same or a similar ceramicmaterial are used. In this case, the pot may serve as a reservoir; thetype of base of the brake disk with the bottom of the pot on the baseensures that the portions of the brake disk later acting as frictionsurfaces do not come into contact with the base and therefore with thereleasing agent layer present thereon, preferably a boron nitride layer.The molded-on pot preferably serves to fasten the brake disk on the hubof the vehicle. In this case, the carbon porous body is in the form of asheath layer of a truncated cone with a cylindrical disk mounted on theexterior of the base and a cover disk on the upper truncated end of thetruncated cone, and the cover disk preferably has a central recess inthe form of a hollow cylinder.

The bodies produced in this way from the ceramic materials arepreferably used as friction disks, in particular for brake disks andclutch driving disks.

The process may thus be implemented as a continuous process, and inparticular the low thermal inertia advantageously influences thecontrollability of the process by limiting the masses that are to beheated and cooled. It is also possible, however, to run thissiliconizing process as a “batch” process.

The process will be explained by the accompanying drawings, of which

FIG. 1 shows a cross-section through a cylindrical porous carbon body 1,which, in contact with a base 2, which is provided with a layer 4 thatconsists of boron nitride, forms a reservoir 3, in which an amount ofsilicon in the form of particles 6 that is sufficient for reaction or acompact body is contained, and

FIG. 2 shows a cross-section through a porous initial body 5 thatconsists of carbon for a carbon-ceramic brake disk with a molded-on potbody 5′ on a base 2 as shown in FIG. 1, and Si particles 6 are alsocontained in the reservoir 3 that is formed from the pot body.

In the illustrated embodiments, silicon is received over the entireinterior jacket surface of the cylinder ring (FIG. 1) or over theinterior surface of the basin-shaped reservoir that is shown in FIG. 2.

Heating of the configuration shown in FIG. 1 is preferably effected byintroduction of energy into the base 2 or especially preferably into theporous carbon body 1 itself; this may be accomplished by, for example,induction of turbulence or by radiation.

In the process, in the bed of the particles 6, the silicon melts intothe reservoir 3 that is formed from the cavity in contact with the base2 and penetrates into the porous carbon body 1 through the inside jacketsurface of the cylinder ring. The carbon in the body 1 reacts with thepenetrated silicon with the formation of silicon carbide. The coating ofthe base 2 with a layer 4 that consists of boron nitride in this casebrings about that after the cooling of the body that is glazed throughreaction to form silicon carbide, it can be dissolved easily from thebase 2.

In the same way, the configuration that is shown in FIG. 2 is heated,and here, a more porous body that consists of carbon 5 is used, which inthe center has a molded-on pot in the form of a jacket surface 7 and thecover surface 8 of a truncated cone. In the center of the cover surface8, there is a recess 9 with a circular cross-section that forms a hollowcylinder. The silicon that is present in the reservoir 3 that is formedin the form of a recess that is open at the top may penetrate the body 5in this embodiment after melting over a surface that is larger incomparison to FIG. 1. The part 1′ of the body, corresponding to thefriction in the brake disk area exposed during braking, may not comeinto contact here with the boron nitride of layer 4; avoidingcontamination with boron nitride is desired for the friction layer of abrake disk.

The effect is illustrated through the following examples:

EXAMPLE 1 Molded Bodies with Threefold Fiber Protection

A prepreg (impregnated fabric) was produced from a fabric of carbonmultifilaments (3 K rovings, i.e., 3000 individual carbon filaments witha surface area-related mass of about 240 g/m²) by impregnation by anaqeous resol. Excess phenol resin was removed by pressing. The fabricwas cut into laminar structures of about 500 mm in diameter, and thelatter were stiffened with intermediate layers of siliconized paper atabout 140° C. in a press under a pressure of about 5 MPa for threehours.

The pressed and stiffened stacks of impregnated fabrics were thencarbonized in a furnace under nitrogen as a cover gas at a temperatureof up to 900° C. In the process it was heated at a rate of about 4 K/hfrom 300° C. up to 600° C. to achieve uniform carbonization. Aftercooling, also under cover gas, the carbonized fabric plates wereimpregnated again with a phenol resin (Novolak, Bakelite IT 491®), driedand in turn carbonized under cover gas for about 8 hours at 950° C.After cooling to room temperature, it was impregnated again, this timewith tar pitch with a softening temperature of about 60° C. according toDIN [German Industrial Standard] 52025. The impregnated fabric plateswere carbonized again at about 950° C. for about eight hours. Then, theplates were heated under cover gas to 2200° C., left at this temperaturefor twenty minutes, and ground in a fly cutter mill with a 5 mm sieveinsert after cooling.

The ground material (2,750 g) was then mixed with a mixture thatconsists of 1,500 g of a phenol resin (resol, Norsophen 1203®, BakeliteCompany) and 450 g of a ground (maximum particle size 20 μm) of coal-tarpitch with a softening temperature of 230° C. according to DIN 52025 atroom temperature (23° C.) in a Z-arm kneader. The homogenized mixturewas completely hardened in a mold in a heatable press at 1.5 MPa (15bar) and a temperature of 150° C. for two hours. The hardened moldedbody was removed and carbonized as above at 900° C.

EXAMPLE 2 Molded Bodies with Simple Fiber Protection

A prepreg (impregnated fabric) was produced from a fabric of carbonmultifilaments (3 K rovings, i.e., 3000 individual carbon filaments witha surface-area-related mass of about 240 g/m²) produced by impregnationwith an aqueous resol. Excess phenol resin was removed by pressing. Thefabric was cut into laminar structures of about 500 mm in diameter, andthe latter were hardened with intermediate layers of siliconized paperat about 140° C. in a press under a pressure of about 5 MPa for threehours. The hardened material was ground in a fly cutter mill with a 5 mmsieve insert.

The ground material (2,750 g) was then mixed with a mixture of 1,500 gof a phenol resin (resol, Norsophen 1203®, Bakelite Company) and 450 gof a ground (maximum particle size of 20 μm) coal-tar pitch with asoftening temperature of 230° C. according to DIN 52025 at roomtemperature (23° C.) in a Z-arm kneader. The homogenized mixture wascompletely hardened in a mold in a heatable press at 1.5 MPa (15 bar)and a temperature of 150° C. for two hours. The hardened molded body wasremoved and carbonized as above at 900° C.

EXAMPLE 3 Siliconization of the Molded Bodies

In each case, 3 molded bodies in the form of cylindrical disks at aheight of 36 mm, an inside diameter of 155 mm and an outside diameter of380 mm, which had been produced according to Examples 1 and 2,

-   -   a) were applied according to the conventional way        (siliconization by attaching the molded body to the wicks that        consist of porous carbon material, which are in contact with a        bath that consists of liquid silicon, heating rate of 10        K/minute, holding time of 30 minutes, temperature about 1600°        C., vacuum), and    -   b) according to the invention, by the cylindrical ring disks        being placed on quadratic graphite plates with an edge length of        450 mm and a boron nitride coating with a thickness of about 0.1        mm, the empty space in the center of the cylindrical ring was        filled with silicon granulate (diameter range about 0.5 mm to 4        mm), and this configuration was heated in an evacuated induction        furnace to 1600° C. during a period of about 4.2 minutes; the        temperature was held for another two minutes.

The differently treated molded bodies were cooled; they were completelysiliconized according to the selected time, i.e., the remainingpercentage by mass of matrix-carbon in the sample was less than 7%.

In these molded bodies, the proportion of carbon fibers that were notattacked in the infiltration with silicon was then determined. In thiscase, the following was produced as a mean value via the molded bodiesexamined:

a) Wicking b) According to process the invention Molded bodies accordingto Example 1 95.1% 96.4% Molded bodies according to Example 2 52.0%95.3%

LIST OF REFERENCE NUMERALS

-   -   1 Cylindrical porous carbon bodies    -   1′ Cylindrical part of the porous carbon bodies 5    -   2 Base    -   3 Cavity as reservoir    -   3′ Recess open at the top as a reservoir    -   4 Boron nitride layer    -   5 Porous body that consists of carbon    -   6 Silicon    -   7 Jacket surface    -   8 Cover surface    -   9 Recess with circular cross-section

1-13. (canceled)
 14. A ceramic article made by the method comprising:providing a base member formed of a material nonreactant with moltensilicon; configuring a carbon fiber, porous body provided with anopening therethrough; positioning said body on said base member so thatthe opening of said body cooperates with a portion of said base memberto form a reservoir; depositing silicon particles in said reservoir; andapplying heat to melt said silicon particles and cause molten silicon toinfiltrate in said porous body and react with said carbon fibers to formsilicon carbide.
 15. A ceramic article according to claim 14 whereinsaid base member is formed of a graphite material.
 16. A ceramic articleaccording to claim 14 including interposing a releasing layer betweensaid base member and said porous body.
 17. A ceramic article accordingto claim 14 wherein said heating is formed by induction heating.
 18. Aceramic article according to claim 14 wherein said heating is performedby radiant heating.
 19. A ceramic article according to claim 14 whereinat least a portion of a side wall of the opening in said porous body isdisposed at other than a right angle to said base member when saidporous body is positioned on said base member for increasing the surfacearea of the porous body to enhance infiltration of molten silicon.
 20. Aceramic article according to claim 19 when said opening and said porousbody is configured so that it cooperates with said base member toprovide a reservoir having a truncated cone configuration.
 21. A ceramicarticle according to claim 14 including configuring said ceramic articleas a friction disc.
 22. A ceramic article according to claim 14including configuring said ceramic article as a brake disc.
 23. Aceramic article according to claim 14 including configuring said ceramicarticle as a clutch disc.
 24. A ceramic article according to claim 16wherein the releasing layer comprises boron nitride.